The Stuck Thermometer
Maya heats a beaker of ice and water on a hot plate turned up high. For more than five minutes the thermometer sits at 0 °C and will not move.
Where is all that energy going?
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Science · Grade 8 Science
Chapter 1: Atoms and the Particle Model
Why the thermometer stops while you are still heating.
Maya heats a beaker of ice and water on a hot plate turned up high. For more than five minutes the thermometer sits at 0 °C and will not move.
Where is all that energy going?
Make your guess to turn the page.
Still to come: 10 pages · Words to Know · What We Learned · 4 questions · Explain It in Writing · Ask and Wonder
The idea this lesson is working on — Students equate heating with getting hotter, so a thermometer that will not rise looks broken or leaky. The book tests both of Ben’s explanations directly: the thermometer is checked against another, and the flat part is shown to grow in proportion to the ice. Ask: if energy were escaping, why would twice the ice keep the line flat twice as long? The idea to reach is that energy can go into separating particles, which a thermometer cannot see.
Changes of state are physical — The substance is the same before and after: ice, water and steam are all water. No new substance appears, and the change reverses completely on cooling, releasing the same energy that separating the particles took in.
About the numbers — Melting ice takes about 334 joules per gram, warming water takes about 4.2 joules per gram per degree, and boiling takes about 2,260 joules per gram, about 6.8 times melting. With a steady supply of about 100 joules a second reaching 100 g of water, those give roughly 5.5, 7 and 38 minutes. Real hot plates lose energy to the room, so classroom times run longer.